1. Field of the Invention
The present invention relates to a battery charger, and more specifically, to a battery charger which prevents the secondary battery of a mobile apparatus from overcharging.
2. Description of the Prior Art
FIG. 2 shows a circuit diagram of the conventional prior art battery charger which charges a secondary battery for a mobile apparatus, for example, a mobile telephone. In FIG. 2, the battery charger includes a charge circuit 2 which provides a charging current corresponding to the current capacity of a secondary battery 6 through a transistor 4 and a diode 5. A timer 3 works from the voltage supplied from an outside battery 22, such as a car battery, and sets the charging time. When the set time expires, the timer 3 outputs a cut off signal to the transistor 4. The transistor 4 then cuts off the charging current and stops charging the secondary battery 6. The diode 5 prevents backward current flow from the secondary battery 6 when the voltage of the secondary battery is higher than the output voltage of the charge circuit 2. The voltage is supplied to the mobile apparatus 100 from the outside battery 22 in the car through the connector 1. A starting motor 21 for starting the car engine is activated by closing a starting switch 23.
The operation of the above conventional prior art is explained hereinafter.
First of all, when the voltage is applied to the charge circuit 2 through the connector 1 from the outside battery 22, the charge circuit 2 outputs a predetermined charging current corresponding to the secondary battery rated current capacity so that the secondary battery 6 is charged optimally through the transistor 4 and the diode 5.
When the voltage is applied to the timer 3, the timer 3 sets the timer counter and the timer counter starts to count the charging time for charging the secondary battery 6. When the timer 3 is set, the timer 3 outputs a low level signal (for example, 0 volt) and activates the transistor 4. By applying the low level signal to the base of the transistor 4 from the timer 3, the current from the charge circuit 2 is supplied to the secondary battery 6 through the transistor 4 and the diode 5 so that the secondary battery is charged. After the predetermined set time is over, the timer 3 outputs a high level signal (for example, 6 volts) to the transistor 4. Then the transistor 4 cuts off the charging current so that overcharging of the secondary battery 6 is prevented.
Since the battery charger uses the car battery as the outside battery, if the starting motor 21 is activated by closing the starting switch 23, the voltage at the connector 1 drops by a large amount for several seconds. The resulting abrupt decrease of the applied voltage to the timer 3 causes the timer counter in the timer 3 to reset and the timer counter starts to count the charging time (usually, 4 hours to 8 hours) from the beginning. Because of this reset of the timer counter, the secondary battery 6 is charged from the beginning of the set time period and the secondary battery becomes overcharged. Because of the overcharging, the lifetime of the secondary battery is greatly reduced.
It is a primary object of the present invention to provide a battery charger which prevents the secondary battery in a mobile apparatus from overcharging, when the voltage of the outside power source happens to drop for several seconds as a result of starting the car engine.